1. Field of the Invention
The invention relates to a method of automatically transferring and injecting a liquid sample with the aid of an injection syringe. The invention also relates to a device for carrying out this method and to an atomic absorption spectrometer which includes said device.
2. Description of the Prior Art
It is known to fill injection syringe manually and expel its contents into a receiver; for example, in the case of a device for atomic absorption spectroscopy, to inject the contents (generally between 1 and 100 .mu.l of sample liquid) into the furnace, for which openings or valves in the furnace housing have to be opened and closed. This method has the drawback that minor differences in the position in which the injection syringe is disposed in the furnace give rise to variations in the measuring results.
In atomic absorption spectrometry it is common practice to automate a number of operations, such as programming of the temperature variation of the furnace. It is also known to fill the furnace automatically and periodically with a sample of the substance to be examined. According to the method known from the journal "Analyst", Volume 97, pages 647-652, the liquid sample is transferred from a first pipe by means of a suitable sliding valve to a second pipe is connected to the furnace by a quartz capillary.
The sample is automatically blown into the furnace with the inert carrier gas argon, which flows through the second pipe. The known method is suitable for the analysis of samples which can be made to flow continuously into the second pipe via the sliding valve. However, the method is not suitable for the examination of sample in vials which contain very small amounts of sample liquid. A further drawback of that known method is that the comparatively low amount of sample liquid must be blown through a comparatively long tube before it can reach the furnace. During transport through said tube liquids may coagulate or foam, as a result of which the samples supplied to the furnace are not readily reproducible. Indeed, cross-contamination may arise owing to extremely small droplets of the sample liquid which are left behind in the tube.
Furthermore, experiments have taught that an injection needle, of which at least the inner wall is made of a metallic material, may affect the metal concentration of the sample liquid if the metal concentration in said liquid is a few parts per million or less. The extent to which said concentration is affected inter alia depends on the element to be analysed, the acidity of the sample and the choice of the metal of the needle. Generally, the effect is appreciable, and, moreover, the magnitude of the effect is not reproducible.
Furthermore, it is not desirable to bring the sample liquids into contact with metal at locations other than the inner wall of the injection needle, for example the metal of the cylinder or the plunger of the injection syringe. When the injection syringe is filled, i.e. when the plunger of the injection syringe is withdrawn to cause the liquid to be drawn into the injection needle, care must be taken that the liquid remains in the injection needle.
Moreover, the liquid sample must be injected completely. In other words: no drop of liquid should be left on the injection needle.